Prolonged neuronal afterdischarge in response to brief stimulation occurs naturally in a variety of cell types, of both vertebrate and invertebrate origin, and finds pathological expression in the phenomenon of epileptic seizures. The research proposed in this application is aimed at understanding the mechanism of neuronal afterdischarge in a model system, the peptidergic bag cell neurons of Aplysia. Previous work has indicated that the onset of repetitive firing and the enhancement of the height and width of action potentials at the onset of a bag cell neuronal afterdischarge is causally related to a rise in cyclic AMP concentrations and to the phosphorylation of substrate proteins. The nature of the change in electricl properties induced by protein phosphorylation will be investigated by microinjection and internal perfusion of the catalytic subunit of cyclic AMP-dependent protein kinase into isolated neurons under voltage clamp conditions, as well as into bag cell neurons in intact ganglia. Changes in electrical properties will be further characterized using single channel recordings on cultured bag cell neurons. To determine which protein substrates are causally related to afterdischarge, the time course of phosphorylation of specific proteins, during afterdischarge, will be determined. Those undergoing changes in concert with the afterdischarge itself will be characterized further. Finally, to investigate whether cyclic AMP-depenbdent phosphorylation may influence electrical excitability through an effect on internal calcium concentrations, direct measurements of intracellular calcium will be made in intact cells, and the effects of phosphorylation on calcium uptake will be determined in membrane preparations of the bag cell neurons.